title: Double sac sign and intradecidual sign in early pregnancy: interobserver reliability and frequency of occurrence.
abstract: OBJECTIVE
  To assess the interobserver agreement, frequency of occurrence, and prognostic importance of the double sac sign (DSS), intradecidual sign (IDS), and other sonographic findings in early intrauterine pregnancies.
  
  
  METHODS
  We retrospectively identified all sonograms obtained between January 1, 2006, and December 31, 2011, in which: (1) the scan demonstrated an intrauterine fluid collection without a yolk sac or embryo; (2) a follow-up scan confirmed an intrauterine pregnancy; and (3) the first-trimester outcome was known. Each coinvestigator characterized the 199 study sonograms as demonstrating or not demonstrating a DSS or an IDS, based on judgment about whether the scan met published criteria defining these signs.
  
  
  RESULTS
  Interobserver agreement was poor for the DSS (κ= 0.24) and IDS (κ= 0.23). Scans frequently demonstrated neither sign: 150 cases (75.4%) if we considered a sign to be present when both investigators graded it as present and 69 cases (34.7%) using the looser criterion that either graded it as present. The presence of a DSS or an IDS was unrelated to the β-human chorionic gonadotropin (β-hCG) value (P > .05, t test, all comparisons). An inner echogenic ring was present in 158 cases (79.4%), and the decidua was brighter peripherally than centrally in 102 (51.3%). The first-trimester outcome was unrelated to the presence of a DSS or an IDS, presence of an inner echogenic ring, or decidual appearance (P > .05, χ(2), all comparisons).
  
  
  CONCLUSIONS
  The sonographic appearance of early gestational sacs, before visualization of a yolk sac or embryo, is highly variable. The DSS and IDS are often absent; there is poor interobserver agreement regarding these signs; and the prognosis is unrelated to their presence or absence. A round or oval intrauterine fluid collection in a woman with positive β-hCG should be treated as a gestational sac until proven otherwise, regardless of whether it demonstrates a DSS or an IDS.
authors.affiliation: Department of Radiology, Brigham and Women's Hospital, 75 Francis St, Boston, MA 02115, USA. pdoubilet@partners.org
authors.forename: Peter M
authors.initials: PM
authors.lastname: Doubilet
authors.forename: Carol B
authors.initials: CB
authors.lastname: Benson
mesh_terms: D000328:Adult; D001900:Boston / Q000453:epidemiology; D005260:Female; D058746:Gestational Sac / Q000000981:diagnostic imaging*; D006801:Humans; D015994:Incidence; D015588:Observer Variation; D011247:Pregnancy; D018873:Pregnancy Rate; D011258:Pregnancy Tests / Q000379:methods* / Q000706:statistics & numerical data*; D015203:Reproducibility of Results; D012307:Risk Factors; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000379:methods / Q000706:statistics & numerical data*
keywords: double sac sign; early gestational sac; intradecidual sign; pregnancy; sonography